As far as I know, there are no direct trains from Cardiff to Euston. In addition to that, it’s far too long for a TSW route.
Route doesn’t go to Cardiff the only London bound train from Cardiff is GWR 800 to Paddington. They’ve only gone to Euston on a diversion route when there’s been engineering works between Reading and Paddington
OP keeps posting vague and in this case nonsensical suggestions for routes. Maybe time the mods had a word with them. Obviously struggling to understand about the rail network or what constitutes a realistic, mature and articulate suggestion.
Cardiff does now have a 221 but it’s Cross Country Cardiff to Edinburgh couples to another 221 at Birmingham New Street, from Penzance/Plymouth and Cross Country won’t give a licence.